1. Field of the Invention
The present invention relates to a multi-phase modulator, and more particularly to a multi-phase modulator which modulates digital orthogonal data and has automatic compensators for offsets of orthogonal adjustment.
2. Description of the Related Art
An exemplary QPSK (quadrature phase shift keying) modulator will be described below with reference to FIG. 8 showing prior art. Data Q 201 and data I 202 for transmission data are re-timed by a timing signal 205 in retiming circuits (RE-TIME) 1 and 2 respectively. Output signals 101 and 102 from the RE-TIMEs 1 and 2 are set to all "1" or all "0" by a modulation/no-modulation switching signal (MOD/NO MOD) 203 during a no-modulation period.
Bandwidths of the output signals 101 and 102 are limited by bandpass filters (digital filters) (BPF) 3 and 4 respectively. Outputs of the BPF 3 and 4 are converted into analog voltage signals 105 and 106 by digital/analog converters (D/A CONV) 5 and 6 respectively. Sampling signals contained in outputs 105 and 106 from the D/A CONVs 5 and 6 are removed by low-pass filter circuits (LPF) 7 and 8. As a result, output signals 107 and 108 from the LPFs 7 and 8 are obtained. Then, the output signals 107 and 108 are differentiated with reference voltages V9 and V10 and amplified by differential amplifiers (DIFF AMP) 9 and 10. Then, output signals 109 and 110 of the DIFF AMPs 9 and 10 are sent to inputs of binary phase-shift-keying modulators (BPSK MOD) 11 and 12.
A local frequency signal (LO FREQ) 204 is divided into signals 113 and 113' having a phase difference of 90 degrees from each other by a 90-degree phase shifter (.pi./2 PHASE SHIFTER) 13. The transmission signals 111 and 112 are 0-.pi. phase-modulated respectively by the BPSK MODs 11 and 12. Then the resulting signal 114 combined by a combiner (COMB) 14 is a QPSK-modulated signal.
Under this situation, the reference voltages V9 and V10 are adjusted and set so that a signal-state space diagram of the QPSK-modulated signal is as shown in FIG. 5. In the signal-state space diagram, all signal-states have equal amplitudes and are evenly distributed (90-degrees spaced) on a circle.
The conventional modulator shown in FIG. 8 has a disadvantage in that it causes offsets of orthogonal characteristics after the completion of the adjustment of amplitude and phase characteristics due to external factors such as the offset change and the gain change of the DIFF AMPs 9 and 10, the phase offset of the .pi./2 PHASE SHIFTER 13, and temperature change. These offsets degrade modulation characteristics and generate some problems of a high bit error rate, a phase ambiguity and so on in a receive side.